NMR equipment capable of applying a radio-frequency signal of high resonance frequency to a sample in uniform high magnetic fields (B0) in order to increase the resolution in nuclear magnetic resonance spectral (NMR spectral) has been developed. To generate high magnetic fields of 10 tesla (T) or higher, a superconducting magnet is generally used. At present, high magnetic field NMR equipment for mainly analyzing the structure of protein is being developed. NMR equipment with magnetic field intensity of 21.6 T and resonance frequency of 920 MHz is produced.
On the other hand, a probe for receiving a free induction decay (FID) signal generated by a sample in response to the applied radio-frequency pulse signal is required to have high sensitivity. In the case of a sample whose amount is small like protein, the intensity of the FID signal is particularly low and it takes long time for measurement. Most noises of the probe occur due to electric resistance of the probe as a component of a resonator and depend on the temperature and surface resistance of a material. To reduce the noises, as disclosed in U.S. Pat. No. 5,247,256, a probe coil and a preamplifier are disposed at a low temperature. In addition, recently, a high-temperature superconductor of which surface resistance is lower than that of a normal metal such as copper by two orders of magnitude or more is used. U.S. Pat. No. 5,585,723 discloses the technique of using a high-temperature superconductor for a probe coil.
A superconducting magnet having excellent uniformity in magnetic field intensity is realized by an integral-type solenoid magnet, and a variation with respect to a sample to be measured is as small as 10−9. In NMR equipment having such a solenoid magnet, the axial direction of a sample tube for introducing a sample to a magnetic field coincides with the axial direction of the solenoid magnet. A radio-frequency signal is applied perpendicular to the axial direction of the sample tube and the solenoid magnet. Consequently, a saddle coil and a bird-cage antenna are used for the probe. Japanese Unexamined Patent Publication No. Hei 11-133127 discloses the technique using a bird-cage antenna.
In the case of making a saddle or bird-cage coil or antenna of a superconductor, particularly, a high-temperature superconductor in order to reduce noise in the probe coil, a superconducting thin film formed on a flat oxide single-crystal substrate is used, so that a shape cannot be chosen freely and it is difficult to efficiently cover the sample. Therefore, although noise of the coil caused by resistance is reduced, the efficiency of application of the radio-frequency signal is reduced due to decrease in the filling factor related to the shape.
The filling factor of the solenoid coil is higher than that of a saddle coil, a bird-cage coil, or a bird-cage antenna. A technique of making a solenoid coil of an oxide high-temperature superconductor is described in the above-described U.S. Pat. No. 5,585,723. In the known technique, a flat donut-shaped ring (one coil part) is fabricated by a high-temperature superconducting thin film, and a part in the ring is trimmed, thereby forming a capacitor via a substrate. In such a manner, an LC resonator is formed. Since it is difficult to electrically connect coil parts, a solenoid coil cannot be formed by connecting a plurality of flat donut-shaped rings. A radio-frequency signal is applied to a sample by inductive coupling using mutual inductance of the coil parts, and an FID signal from the sample is received. However, in the method of using the mutual inductance of the coil parts, adjustment is difficult and, generally, efficiency is low.
To increase the filling factor and the efficiency of detecting the FID signal, it is sufficient to increase the number of turns of the solenoid coil. To realize it, an NMR probe for radio frequency accompanies difficulties. Specifically, although it is necessary to make the product between the inductance of a coil and the capacitance of a capacitor at a feeding point correspond to the radio-frequency to be applied, it is difficult to set the capacitance of the capacitor at the feeding point to 3 pF porless from the viewpoint of manufacture. Further, in NMR equipment in which a radio-frequency signal used for analysis of the structure of protein is in the class of 600 MHz, it is strongly demanded to reduce the inductance of the coil. Consequently, manufacture becomes more difficult.